I think the creator of this topic meant a different feature: every app can register url schemes. For example, Textastic could register a textastic:// url scheme. This would allow you to use Safari to open files/urls with Textastic.

Hello,

Textastic 2.2.1 came with a syntax definition for SecondLife LSL. 

 
In Textastic 3.0 it's not built-in, but you can get a TextMate bundle from http://digilander.libero.it/usemac/tm.zip and add it to Textastic yourself:

http://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html

I've found the link to this TextMate bundle for LSL in the following SecondLife forum thread: http://forums-archive.secondlife.com/54/ae/89389/1.html It seems to be well-maintained.

There is also another one in TextMate's SVN repository at http://svn.textmate.org/trunk/Review/Bundles/SecondLife%20LSL.tmbundle/

Hello,

Textastic 3.0 uses TextMate syntax definitions and themes for syntax 
highlighting.

Now you can also add your own syntax definitions. This is explained in 
the manual athttp://www.textasticapp.com/v3/manual/lessons/How_can_I_add_my_own_syntax_definitions_and_themes.html

Here is a bundle for Verilog: 

VHDL: 

https://github.com/bpadalino/VHDL.tmbundle